And I'm Melissa Block. An emphatic message today about Syria from top Pentagon officials: Military options are not the best answer. Some Republican lawmakers, most prominently John McCain, say it's time to launch targeted airstrikes, just as NATO forces did in Libya last year. But as NPR's Larry Abramson reports, military leaders insist the Syrian case is different.

LARRY ABRAMSON, BYLINE: This week, the administration has been busy explaining why it is not ready to attack Iran. And today before the Senate Armed Services Committee, Defense Secretary Leon Panetta made his case for restraint regarding Syria.

SECRETARY LEON PANETTA: Although we will not rule out any future course of action, currently, the administration is focusing on diplomatic and political approaches rather than military intervention.

ABRAMSON: Panetta repeated the administration's belief that the Syrian government of Bashar al-Assad is bound to fall eventually. But some Republicans said that is not certain, and in the meantime, thousands have died in nearly a year of protests and violent crackdowns. Republican Senator John McCain said there's only one way to give the Syrian opposition some breathing space: a U.S.-led airstrike.

SENATOR JOHN MCCAIN: Which could break Assad's siege of contested cities in Syria and help the opposition to Assad on the ground to establish and defend safe havens in Syria where they can organize and plan their political and military activities against Assad.

ABRAMSON: McCain said NATO airstrikes worked in Libya just last year, and they worked in the Balkans in the 1990s. But Secretary Panetta and General Martin Dempsey, chairman of the Joint Chiefs of Staff, said the situations are very different. First, Dempsey said, the Syrians have an advanced air defense system.

GENERAL MARTIN DEMPSEY: They have approximately five times more air - more sophisticated air defense systems than existed in Libya covering one-fifth of the terrain.

ABRAMSON: A smaller country, better air defenses, General Dempsey said that means an air campaign against Syria would require lots of aircraft to knock out those defenses. Both men said military and political issues are intertwined in Syria. They insisted the opposition there is still fractured, making it hard to know exactly which groups the U.S. would be helping. That could lead to unintended consequences. Panetta said U.S. involvement could aid extremist groups in Syria - a scary proposition considering that country's chemical weapons arsenal.

PANETTA: They've got huge stockpiles, and that if it got into the wrong hands, it would really be a threat to the security not only of the regional countries but to the United States.

ABRAMSON: None of this dampened Senator McCain's desire that the U.S. do something. He pointed out that while the U.S. debates, Syria is getting help from its key ally - Iran. Listen to what McCain said when committee chair Carl Levin asked General Dempsey a question.

SENATOR CARL LEVIN: Are you able to tell us what Iran is supplying?

DEMPSEY: I can in close session.

MCCAIN: Can you quote from The Washington Post?

ABRAMSON: McCain is citing a recent Washington Post article that said Iran is ramping up lethal aid to Syria according to anonymous sources. Syrian government forces have been on the offensive recently, breaking a siege in a key town. McCain said if the U.S. simply waits for the regime to fall, it may not happen.

MCCAIN: Assad needs to know that he will not win, and unfortunately, that is not the case now.

ABRAMSON: Panetta and Dempsey insisted they are not sitting on their hands. General Dempsey said the Pentagon has drawn up a list of options.

ABRAMSON: With airstrikes way at the end of that list. Both men emphasized that any military action would require a coalition effort that includes Arab nations. That coalition, they said, is still not in place. Those eager to see results in Syria have more waiting ahead. Larry Abramson, NPR News, Washington. Transcript provided by NPR, Copyright NPR.